For the surface treatment of substrates, many treating agents are used for various purposes. The agent and method for use in surface treatment are generally selected in accordance with the surface properties of a substrate to be treated and other factors. For example, lipophilic treatment with lubricants and metal soaps, hydrophilic treatment with surfactants and water-soluble polymers, and water/oil repellent treatment with silicone fluids are known.
Herein hydrophilic treatment is of concern. When powder particles are dispersed in an aqueous medium, for example, particles tend to agglomerate together due to the electricity and polarity on particle surfaces, trace impurities, and the like. Prior attempts to minimize agglomeration in aqueous media include hydrophilic treatment on particle surfaces. However, hydrophilic treatments with conventional surfactants and water-soluble polymers are not fully effective. When a composition is loaded with powder treated with such an agent, there is a likelihood that the treating agent may dissociate from particles in the system, leading to color variations, a color difference between appearance and coat, and a decreasing degree of dispersion with time, which is detrimental to the applicability of the composition. Depending on the identity of surfactant, the composition becomes irritant to the skin and thus undesired for use in cosmetics.
JP-A H09-2815 discloses that the surface of smectite clay mineral is rendered hydrophilic by treatment with a polyether-modified silane compound having a hydrolyzable silyl group. Since the hydrolyzable silyl group forms a chemical bond with the particle surface, this method overcomes the problem that the treating agent may dissociate from particles in the system. The method achieves a degree of hydrophilicity which is still unsatisfactory. For example, when the method is applied to the surface treatment of titanium oxide powder, the treated powder is not fully dispersible in aqueous media.